def isValid(side1,side2,side3):
    if side1 > side2:
        side1,side2 = side2,side1
    if side2 > side3:
        side2,side3=side3,side2
    return side2+side1 > side3

def area(side1,side2,side3):
    if isValid(side1,side2,side3):
        s = (side1+side2+side3)/2
        return "此三解形的面积为："+str(format((s * (s-side1) * (s-side2)*(s-side3) )**0.5,".3f"))
    else:
        return "Input is invalid"

def main():
    side1,side2,side3 = eval(input("请输入三角形的三条边长："))
    print(area(side1,side2,side3))

main()